Refracting prisms (Item No.: P )

Transcription

1 Teacher's/Lecturer's Sheet Refracting prisms (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Reflexion und.rechung Experiment: Umlenkprisma Difficulty Preparation Time Execution Time Recommended Group Size Intermediate 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: White paper (DIN A4) Ruler (approx. 30 cm) Keywords: Task and equipment Information for teachers Additional Information The goal of this experiment is that the students become acquainted with special cases of the refraction of light at a prism which are important in technical applications (optical instrument construction). With the investigation and explanation of the light path as a function of the position of the prism in relation to the light source, knowledge about the laws of reflection and refraction, as well as total reflection, are strengthened. In the first part of the experiment, the normal refraction at a prism is investigated (as a revision exercise); whereas in the second and third parts of the experiment the possibility of employing a right-angled prism at 90 or 180 to reverse the incident light is recognised and understood. It can also be recommended to take advantage of group work: each group investigates the path of light with a different position of the prism and, at the end of the experiment, the results and the conclusions are collected for a total result. Suggestions Careful preparation of the A4 sheet of paper (crossed lines at an angle of 90 ) and conscientious adherence to the suggestions for execution will ensure success in the experiment. In particular, care should be taken that the rough side of the prism is always underneath so that the passage of the light beam inside the prism is visible. The marking of the outline of the prism in the individual parts of the experiment allows not only the re-adjustment after inadvertent movement of the block, but also the complete sketching of the light path in the evaluation phase. Otherwise, difficulties could arise for the students since a precise marking within the prism is not possible. Marking the point at the side of the prism where total reflection takes place is a possible aid.

2 Refracting prisms (Item No.: P ) Task and equipment Task Not all prisms are the same Investigate the course of a light beam on emergence from a right-angled prism when the light falls at an angle to the hypotenuse hits a cathetus at right angles is incident to the hypotenuse at right angles.

3 Equipment Position No. Material Order No. Quantity 1.lock, rectangular triangle Light box, halogen 12V/20 W with single-slit/double-slit aperture 1 Additional material PHYWE power supply DC: V, 2 A / AC: 6 V, 12 V, 5 A White paper (DIN A4) 1 Ruler (approx. 30 cm) 1

4 Set-up and procedure Set-up On a sheet of paper, accurately draw right-angled, crossed lines at the top, in the middle and at the bottom (Fig. 1). Fig. 1 Lay the rectangular block with the rough surface underneath with the two catheti legs exactly on the upper intersection. Carefully mark the outline of the prism with thin pencil lines (Fig. 2). Insert the single-slit aperture in the light box on the lens side and position it at the edge of the sheet of paper. Fig. 2 Procedure Connect the light box to the power supply (12 V AC) and switch it on (Fig. 3).

5 Fig. 3 Move the light box until the narrow light beam travels parallel to the optical axis and about 2 cm away from it (Fig. 4). Observe the course of the narrow light beam inside and outside the prism. Note your observations in the table in the report. Mark the course of each light beam in front of and behind the prism with two crosses. Fig. 4 Change the arrangement of the setup to that shown in the picture (using the middle intersection) (Fig. 5). Now, one cathetus is facing the light box. Again carefully mark the outline of the prism with thin pencil lines. Move the light box until the narrow light beam travels parallel to and about 2 cm away from the optical axis. Observe the course of the narrow light beam inside and outside the prism. Note your observations in the report. Again mark the course of each light beam in front of and behind the prism with two crosses. Fig. 5

6 Again change the setup as pictured (Fig. 6). The hypotenuse of the prism must now face the light box. Carefully mark the outline of the prism with a pencil. Move the light box until the narrow light beam is about 1 cm away from and parallel to the optical axis and hits the hypotenuse of the prism. Repeat the observations and mark the light path for this positon of the prism. Fig. 6 Switch off the power supply and remove the light box and the prism from the paper. For each case connect the markings so that the path of the light shining through the prism is visible.

7 Report: Refracting prishs Result - Table 1 Note your observations in the table. Light incident to the prism Observation of the light path inside and outside the prism At an angle to the hypotenuse Perpendicular to a cathetus Perpendicular to the hypotenuse Evaluation - Question 1 For each of the three different positions of the prism, connect the respective crosses so that the course of the light beam outside and also, after drawing the corresponding connections, inside the prism are visible. Once more, compare it with your observations on the paths of the light beams (Table 1).

8 Evaluation - Question 2 Complete the sketch for the first position investigated of the prism (light incident at a slanted angle to the hypotenuse) by drawing in the perpendicular line of incidence and try to explain the course of the narrow light beam, based on your observations and with the help of the law of refraction. Evaluation - Question 3 Compare the three experimental results in terms of the similarity in the paths of the light beams. Give an explanation for the course of the light.

9 Evaluation - Question 4 The right-angled prisms you used to observe the path of light for perpendicular angles of incidence on the hypotenuse or cathetus are called refracting prisms. Suggest some applications for such a prism in technology.